1. Field of the Invention
The present invention relates to an image-receiving sheet for thermal transfer recording and particularly to a thermal transfer image-receiving sheet which stably exhibits excellent antistatic properties and is suitable for sublimation dye transfer recording.
2. Background Art
Various thermal transfer recording methods are known in the art. Among others, a thermal sublimation transfer recording method has recently drawn attention. In this method, a thermal transfer sheet comprising a thermal transfer layer containing a sublimable dye provided on a support such as a polyester film is heated by means of a heating medium such as a thermal head or a laser beam to form an image on the thermal transfer image-receiving sheet. The thermal dye transfer recording method is used as information recording means in various fields. According to the thermal dye transfer recording method, full-color images can be formed in a very short time, and high-quality images comparable to full-color photographic images having excellent reproduction of intermediate colors and gradation can be provided.
On the image-receiving face provided are a receptive layer formed of a thermoplastic resin, for example, a saturated polyester resin, a vinyl chloride-vinyl acetate copolymer, a polycarbonate resin or the like from the viewpoint of receiving a sublimable dye being transferred from the thermal transfer sheet and holding the formed image, and, if necessary, an intermediate layer between the substrate sheet and the receptive layer. Intermediate layers include, for example, a layer for imparting cushioning properties, in the case where a high-rigidity substrate sheet such as PET is used, and a layer for imparting antistatic properties. If necessary, a backside layer formed by coating a composition prepared by adding, to a binder such as an acrylic resin, an organic filler of an acrylic resin, a fluororesin, a polyamide resin or the like, or an inorganic filler such as silica may be formed on the backside from the viewpoint of preventing curling and improving slipperiness.
In the case of the so-called “standard-type thermal transfer image-receiving sheet,” in use, the image-receiving sheet is viewed by taking advantage of reflected light rather than transmitted light. Also in this case, opaque, for example, white PET, expanded PET, other plastic sheets, natural papers, synthetic papers, or laminates of these materials and the like are used as the substrate sheet. Further, the so-called “seal-type thermal transfer image-receiving sheet” is also used in various applications. The seal-type thermal transfer image-receiving sheet comprises a substrate sheet, a receptive layer provided on one side of the substrate sheet and, provided on the other side of the substrate sheet in the following order, an adhesive layer and a release paper using a pressure-sensitive adhesive or the like. This seal type is used in such a manner that an image is formed on the receptive layer by thermal transfer, the release paper is separated, and the sheet is then applied to any object.
The formation of an antistatic layer formed of a surfactant or the like on the surface of a thermal transfer image-receiving sheet is known. This method suffers from problems including that the thermal transfer image-receiving sheet becomes sticky, the antistatic agent is transferred from the top surface onto the backside, and the antistatic agent is transferred onto a carrier roll or the like of a thermal printer. Further, these problems lead to a deterioration in antistatic effect with the elapse of time. There is an alternative method in which an electrically conductive layer is formed of an electrically conductive agent, for example, an electrically conductive carbon black or a metal oxide such as tin oxide, and a binder. In order to impart electrically conductive properties by these electrically conductive agents, a considerably large amount thereof should be added. Further, these electrically conductive agents are in many cases inherently colored ones such as black electrically conductive agents. Therefore, basically, when they are used in image-receiving sheets, the whiteness of the image-receiving sheet is lowered making it impossible to use them.
The formation of an antistatic layer formed of acrylic resins having a quaternary ammonium base has also been proposed as a method for solving the above problems. Specifically, Japanese Patent Laid-Open No. 139816/1990 proposes the provision of an antistatic layer, formed of these materials, between the receptive layer and the substrate. Since, however, these materials have poor waterfastness, even when they are used in this way, the coating strength is remarkably deteriorated under high-humidity and/or high-temperature conditions, particularly high-temperature conditions. This poses problems such as breaking of a coating due to friction with a roll during carrying at the time of printing. Further, these materials basically have poor adhesion to the substrate or other resins. Therefore, materials usable herein are considerably limited. Furthermore, the antistatic properties disadvantageously vary depending upon the environment.
Japanese Patent Laid-Open No. 78255/1999 proposes the use of titanium oxide having a surface which has been treated with an electrically conductive material. Since, however, the particle diameter of the electrically conductive material is not less than 1 μm in terms of major axis, the glossiness of the image-receiving paper surface is disadvantageously lowered. Further, since the electrically conductive material used in the surface treatment is a material having a relatively deep color tone such as tin oxide, even when titanium oxide having an inherently white color is used, the color tone is changed to steel-blue upon treatment for rendering the material electrically conductive. As a result, disadvantageously, the whiteness of an image-receiving paper using this electrically conductive material is somewhat deteriorated.